1. Field of the Invention
The present invention relates to a toroidal current transformer transistor driver, and more particularly, to a toroidal current transformer transistor driver for an electrical ballast for non-lineal limitations from large current variations in primary.
2. Description of the Related Art
Several designs for toroidal current transformer transistor drivers have been designed in the past. In most ballast systems, different approaches are used to drive half bridge or H bridge transistors. One of these approaches involves the use of a toroidal current transformer to drive transistors. The problem arises where the voltage induced in the secondary of a toroidal current transformer fluctuates over an extremely large range. Therefore, the use of toroidal current transformers, without the use of complicated correcting ICs, has been limited to low power ballasts or medium power with stabilized DC source. It has been found that there are differences in current amplitude of 5-7 times (minimum amplitude with respect to maximum amplitude). To protect the gates of the transistors being driven, resistors and zener diodes are used, in addition to other current/voltage limiting circuitry. The energy dissipated by these resistors and zener diodes is wasted and, not infrequently, the generated heat adversely affects the operation of the ballast circuit. Thus, the desirability of limiting the wasted energy to drive the transistors is quite evident.
The current through a ballast's load, typically a halide lamp, is not constantly causing variations in the voltage induced in the secondary of a toroidal current transformer connected to the transistor's gates. These gates require between 12 and 15 volts. Therefore, the number of turns in the secondary need to ensure at least 12 volts output. But when the load current increases seven to ten times the lowest intensity (capable of still driving the transistors), a corresponding secondary voltage increase follows, requiring the usage of limiting voltage elements, such as zener diodes and resistors, to protect the transistor's gates. The resulting heat is wasted energy that tends to adversely affect the operation of the rest of the circuit and shortens its life. Thus, there is a need to find a self-limiting device that is passive to minimize costs and extend the life of the ballast without wasting electrical energy.
One approach in the construction of ballasts is to use circuits with passive valley fill, such as the one commercialized by International Rectifier (El Segundo, Calif.) under part No. IR 2156 and explained in application note AN-1074. The advantage of the IR circuit is that it provides a stable output in the secondary with variation in the load current. However, the use of integrated circuits is expensive and not reliable, especially for high power lamps. The relatively high temperatures to which the ballasts are exposed diminish the life of the IC whereas passive elements, such as a toroidal transformer, have considerably longer lives. To improve this IR IC solution, the present invention provides a passive circuit for limiting the voltage output while variations of the load (lamp) current is used as the primary of the toroidal current transformer.
Other documents describing the closest subject matter provide for a number of more or less complicated features that fail to solve the problem in an efficient and economical way. None of these patents suggest the novel features of the present invention.